It has been estimated that rheumatologist-diagnosed Rheumatoid arthritis (RA) is seen in up to 2% of the users of VA healthcare. Moreover, compared to RA in women, RA in men (the primary VA demographic) leads to greater morbidity, a higher frequency of extra-articular manifestations, and worse disease-related outcomes. With an aging and predominantly male veteran population, it is anticipated that the burden posed by RA will grow dramatically in the VA in coming years. This is highly relevant given the association of RA with; substantially higher mortality risk among veterans, in addition to high rates of work-related disability, and annual societal costs approaching $40 billion in the US alone. Over the past few years, there have been substantial advances in our understanding of RA pathogenesis. Anti-citrullinated protein antibody (ACPA) is highly specific to RA with recent studies suggesting that ACPA are pathogenic with seropositivity portending a poor prognosis including more rapid joint destruction. However, the mechanism(s) by which citrullinated proteins/ peptides are recognized and processed and presented in the context of co-stimulatory molecules is still not well understood. Studies have shown that a unique post-translational modification of proteins that occurs under oxidative stress by malondialdehyde (MDA) and acetaldehyde (AA), termed MAA, up- regulates MHC Class II, increases co-stimulatory molecules and generates cytotoxic and pro-inflammatory responses in the absence of exogenous adjuvant. For the first time, our group has shown that MAA modified proteins are detected in synovial tissues of RA patients and co-localize with citrullinated antigen. Additionally, anti-MAA antibody isotypes are independently associated with ACPA concentration (p < 0.0001) in patients with established RA. Therefore, our overarching hypothesis is that these two post-translational modifications (MAA modification and citrullination) act in concert to drive tolerance loss resulting in the anti-citrulline autoimmune responses characteristic of RA. To investigate this hypothesis, studies in Aim 1 will evaluate adaptive immune responses (autoantibody and T cell responses) to citrullinated and/or MAA-modified proteins. We anticipate that compared to antigens that are only citrullinated or only MAA modified, immune responses to co-modified proteins will be higher in mice following immunization and in RA patients. As prior data has demonstrated that SRs mediate the biological effects of MAA-modified proteins on APCs and other cells, Aim 2 is designed to identify the specific SRs that mediate the effects of citrullinated and/or MAA-modified proteins in RA. In Sub-Aim #1, studies will leverage Chinese Hamster Ovary (CHO) cell lines that are already available in our laboratory and that have been transfected with each of the various SRs. Results of initial experiments will inform the design of subsequent binding studies using human APCs (that simultaneously express multiple SRs) and cell types unique to synovial tissue. Using available SR inhibitors alone or in combination will aid in defining which receptors are found on each cell type. In Sub-Aim #2, will focus on the biological effects (fibrosis, inflammation, calcium influx, PAD expression and citrullination) of the binding of MAA modified and/or citrullinated proteins to the different cell types. Thus, the innovative aspect of this proposal is its focus on the melding of two naturally occurring biological events (citrullination and oxidative stress) that conspire to initiate highly specific autoimmune responses, promoting the development and progression of RA.